1. Field of the Invention
The present invention relates to a cellular mobile telecommunications system and, more particularly, to a radio link control method capable of adaptively controlling the directivity of an antenna of a base station on a mobile station basis and thereby allowing the base station and a plurality of mobile stations to communicate at the same time over channels of the same frequency or nearby frequencies which are apt to interfere with each other.
2. Description of the Related Art
A car telephone system or similar large capacity mobile radio telecommunications system having a plurality of base stations each covering a respective service area is conventional and generally referred to as a cellular telecommunications system. This kind of system allows the same frequency to be repeatedly used between base stations which are free from interference, thereby promoting the effective use of limited frequencies. A cellular telecommunications system is provided with either an omni architecture or a sector architecture, as well known in the art. In the omni architecture, a single non-directional antenna is installed in each base station while, in the sector architecture, a plurality of directional antennas are installed in a single base station. Today, the sector architecture is predominant over the omni architecture since it can reduce the size of a service area, i.e., cell size and can enhance the efficient use of frequencies in space without increasing the number of base stations.
Japanese Patent Laid-Open Publication No. 62-92630, for example, discloses a cellular telecommunications system having the sector configuration and capable of promoting the efficient use of frequencies. In this system, a base station is provided with an antenna having a high front-to-back ratio (ratio of a forward radiation level to a backward radiation level in a directional antenna). Such a base station allows different sector cells thereof to reuse the same frequency.
It has been reported that adaptively controlling directivity in matching relation to the state of a mobile station is advantageous over using a base station antenna having fixed directivity. A method of controlling antenna directivity in a horizontal plane is taught in, for example, a paper entitled "Improving Frequency Utilization Radio by Controlling Antenna Directivity of Cellular Base Station", the Institute of Electronic Data Communication Engineers of Japan, Spring Convention, Fascicle No. 2, B-397, page 2-398, Apr. 1993. The method taught in this paper uses a base station antenna implemented as a circular adaptive array of eight antenna elements. The eight elements each has the feed point thereof adaptively controlled, so that a null point is directed to an interference station. With this kind of method, it is possible to reduce the number of repetitive cells to one. Further, Japanese Patent Laid-Open Publication No. 4-320122 proposes a method of controlling antenna directivity in a vertical plane. This system controls the feed phase of each antenna element constituting a base station antenna, thereby tilting a main beam direction. As a result, the ratio of a received level to an interference level is maximized to reduce interference. This is a promising approach to reduce the repetition distance of the same frequency.
The first-mentioned scheme uses a base station having fixed antenna directivity and uses the same frequency twice or more in a single base station. The second and third schemes provide each base station with an antenna adaptively controllable on a mobile station basis, thereby reducing the distance between base stations which can use the same frequency. These conventional schemes readily suggest, in combination, that a base station antenna having directivity adaptively controllable on a mobile station basis may be used to reduce the distance between base stations which can share the same frequency, while allowing each base station to use the same frequency twice or more. However, when the base station having an antenna whose directivity is adaptively controllable uses the same frequency or nearby frequencies twice or more, interference is apt to occur within the base station even if the base station is free from interference as confirmed at the beginning of communication. As mobile stations communicating with the base station by use of the same frequency or nearby frequencies approach each other with respect to the direction to the base station, antenna directivities using the same frequency or nearby frequencies overlap each other and cause interference to occur.